1. Technical Field of the Invention
The present invention relates to a change-over apparatus for changing over cooling gas passages in a vacuum heat treating furnace.
2. Description of the Related Art
A vacuum heat treating furnace is the one in which inert gas or the like is refilled after depressurization of the inside thereof, in order to heat-treat an article to be treated. Since the vacuum heat treating furnace may completely remove moisture or the like sticking to the interior of the furnace and to the treated article after heating, by depressurizing again the furnace after evaporation of the moisture or the like, and by refilling the inert gas or the like thereinto, there may be exhibited such an merit that heat-treatment may be made without coloring by moisture (it is referred to as the “bright heat-treatment”).
A gas-cooled vacuum heat treating furnace can exhibit several advantages as set forth in the following. Namely, (1) bright heat treatment can be achieved; (2) no occurrence of decarbonization and carburization; (3) occurrence of less deformation; and (4) an acquirement of a satisfactory working environment. However, early gas cooled vacuum heat treating furnaces were of depressurized cooling type, and accordingly, they offered such a disadvantage that its cooling rate cannot be sufficiently high. Thus, there has been materialized rapid circulating gas cooling type furnaces in order to aim at increasing the cooling rate.
Referring to FIG. 1, which is a view illustrating a configuration of a rapid circulating gas cooling type furnace disclosed in non-patent publication 1, i.e., “Vacuum Heat Treatment for Metals (2)” written by Katsuhiro Yamazaki, Heat Treatment Vol. 30-2, April, 1990. In the figure, there are shown a heat-insulator 50, a heater 51, an effective working zone 52, a furnace body 53 with a water cooling jacket, a heat-exchanger 54, a turbofan 55, a fan motor 56, a cooling door 57, a hearth 58, a gas distributor 59, and a damper 60 for changing over cooling gas passages.
Further, patent publication 1, i.e., Japanese Patent Laid-Open Publication No. H5-230528 discloses a method of promoting gas circulation and cooling in a vacuum furnace, as shown in FIG. 2, which comprises a heating chamber 66 provided in a gas tight vacuum container 61 and surrounded by a heat insulation wall 67, an article 64 to be heated being heated in a vacuum atmosphere by a heater 72 disposed in the heating chamber, and a cooler 62 and a fan 63 provided in the gas-tight vacuum container 61, for cooling unoxidizable gas through the cooler 62 and circulating the unoxidizable gas in the heating chamber 66 by the fan 63 through openings 68, 69 formed in the surface of the insulation wall 67 in order to forcibly cool the article 64 to be heated with the circulation gas, wherein a heat-resistant cylindrical hood 65 flared at least its one end is arranged so as to surround the article 64 arranged in the heating chamber 66, while keeping an appropriate distance therebetween, and so as to permit its both ends to be opposed respectively to the openings 68, 69 to thereby urge the unoxidizable gas to circulate in the heating chamber 66. It is noted that in this drawing figure, a damper 70 is provided for changing over cooling gas passages.
As stated above, in the case of adopting a high speed circulation gas cooling type in order to have the benefit of several advantages exhibited by a gas cooling type vacuum heat treating furnace and to enhance the cooling rate, it is extremely important to change over gas passages from the viewpoint of equalizing cooling rates of articles to be treated during cooling thereof.
Thus, in the furnace of high speed circulation gas cooling type as disclosed in the above-mentioned documents, there have been usually employed upper and lower damper units as a mechanism for changing over between upward and downward gas passages. However, in the case of the change-over mechanism employing the upward and downward gas passage, there have been arising problems as indicated below:
(1) Since the damper unit causes large variation in load depending upon its open and close positions, due to a wind pressure of gas which passes through these positions at a high speed, and accordingly, it is difficult to smoothly actuate such a damper unit under affection by the wind pressure in the case of a high pressure gas;
(2) Since the damper unit has such a configuration that an angle from its opening position to its closing position is not in proportion to its opening area, balancing between opening areas is difficult to obtain at the stage of changing over of a plurality of upper and lower drives so that a difference occurs between opening areas of a suction port and a discharge port, or the difference becomes large, and accordingly, the volume of cooling gas varies, resulting in making it difficult to stabilize gas cooling;
(3) Since there exist a plurality of upper and lower dampers, a plurality of drives is necessarily required, and accordingly, the structure of the mechanism becomes complicated; and
(4) Since the opening areas are delimited by the upper and lower dampers, the opening areas are smaller in comparison with the inner surface of the body of the furnace.